The invention relates to a double-walled tailpipe for an exhaust pipe of a motor vehicle exhaust system with an inner shell on which an outer shell is supported in spaced relationship so as to permit air to flow through the gap between the inner and the outer shells and which can be mounted onto an exhaust pipe.
Double-walled tail pipes are known for example from DE 196 51 608 C2 and the corresponding EP 0 848 144 A1. The inner and outer shells are interconnected at the discharge end by engagement of the inner shell with the inwardly bent end edge of the outer shell to which the inner shell is welded. Otherwise, the inner and outer shells are not in contact with each other. The tail pipe is mounted onto the exhaust pipe by sliding the inner shell onto the exhaust pipe. Between the inner and outer shells, there is an annular space, which is open at the front end in the travel direction of the vehicle. At the rear exit end of the annular space, the tail pipe is provided with outlet slots, which are provided in the inwardly bent edge of the outer shell and extend annularly. In the embodiment shown, the heat transfer from the exhaust pipe of the exhaust system to the tail pipe mounted thereon is reduced in that the inner shell is supported by the exhaust pipe only by longitudinal and transverse projections which extend around window-like openings so that direct communication with the air gap is provided for and the heat radiated from the exhaust pipe is carried away partly by the air flowing through the air gap. However, at the exhaust end of the tail pipe, and in the visible area thereof, the inner and outer shells are directly interconnected so that the end section of the tail pipe may be heated in spite of the air flow through the space between inner and the outer shells and the cooling effect achieved thereby. Such heating, particularly at the end of the tail pipe, may lead to undesirable visible discoloration of the tailpipe also in the area of the outer shell. In addition, the connection between the inner and the outer shells as provided by this arrangement is critical also from a manufacturing point of view because, at its location exposed to an observer, it requires optically perfect manufacturing of the connection.
DE 78 38 091 U1 discloses a single wall tail pipe, that is a shield tube, which can be inserted axially onto an exhaust pipe and supported thereon by way of a spring structure, wherein the support, at the same time, fixes the tail pipe axially with respect to the exhaust pipe. The spring support is provided by a spring strip provided with spring webs which extend therefrom outwardly and are disposed in the overlapping area with the exhaust pipe and provide support on the exhaust pipe and also allow an adaptation to different exhaust pipe diameters. In this way, the tail pipe can be mounted onto exhaust pipes of a certain diameter range. However, the air gap between the exhaust pipe and the tail pipe, depends on the respective diameter difference so that the air flow through the air gap may not be sufficient for appropriate cooling of the tail pipe.
It is the object of the present invention to provide a double-walled tail pipe of the kind as discussed above which however is easier to manufacture and which provides for improved cooling so that a discoloration of the tail pipe at least in the visible area, that is, of the outer shell of the tail pipe is avoided.
In a double-wall tail pipe for an exhaust pipe of a motor vehicle exhaust system comprising an inner shell to be mounted on the exhaust pipe, an outer shell surrounding the inner shell and being supported thereon in spaced relationship to form an annular air space therebetween for conducting cooling air therethrough, a support structure provided with air passages is disposed between the inner and the outer shells in an area where the inner shell is seated on the exhaust pipe when the tail pipe is mounted on the exhaust pipe, which support structure interconnects the inner and outer shells in an area which is spaced from the discharge end of the tail pipe.
In the tail pipe according to the invention, the connection between the inner and the outer tubular shells is disposed in an area in which the inner and outer shell are disposed on the exhaust pipe, that is, remote from the visible outer end of the tail pipe, and where the inner shell is supported on the exhaust pipe. The support is such that it includes air inlet openings which are disposed in the air gap or which provide for communication with the air gap.
In a preferred embodiment, which is also easy to manufacture the inner shell is provided, at its upstream end, with a radially outwardly extending flange by which it is supported on, and connected to, the exhaust pipe. The flange includes openings and engages, with its radially outer end, the inner wall of the outer shell. The flange may be an annular flange provided with openings but preferably, it is formed by circumferentially spaced, bent straps provided with mounting sections adapted in shape to the contour of the outer shell. Each strap has an angled end portion disposed adjacent the support portion which bridges the space between the inner and the outer shells. The bent straps preferably have, in a radial cross-section, a Z-shaped contour wherein the outer leg of the z forms the end portion of the strap with which it is mounted to the outer shell. The web of the z forms the support section and the inner leg of the z is formed by the area of the inner shell which is disposed on the exhaust pipe. The axial length of the overlapping area of the inner shell with the exhaust pipe may be limited for example by a stepped structure or by a stop provided on the inner shell.
The straps may include backwardly bent end portions so that they form U-shaped supports disposed between the exhaust pipe and the outer shell, wherein the free leg portion is supported on the exhaust pipe. In this way, a simple support structure may be provided which allows for some relative movement.
Since the support of the inner shell with respect to the outer shell is disposed in spaced relationship from the end of the overlap area with the exhaust pipe the support structure for the inner shell in the outer shell is disposed in the entrance area of the air gap so that the support structure is exposed to the lowest possible air temperature.
Furthermore, the overlap area between the inner shell and the exhaust pipe can be relatively short if, as in a particular embodiment, the outer shell extends at least in some area axially beyond the inner shell and is mounted in that area to the exhaust pipe for example by a bolt.
Such an additional mounting structure may be disposed in an area, which is not normally visible for example at the underside of the exhaust pipe. It is also within the scope of the invention to mount the inner shell on the exhaust pipe by providing securing means in the area of overlap between the inner shell and the exhaust pipe. This can be done for example by means of one or more clamping screws, which extend at least through the inner shell for engagement with the exhaust pipe. Such clamping screws may be provided also in the area of the flange, which supports the outer shell on the inner shell.
With the support arrangement according to the invention with a support of the inner shell on the outer shell in the overlap area with the exhaust pipe, preferably somewhat upstream of the support structure between the inner and outer shells, the support structure may not extend radially, but it may be inclined whereby the support straps become longer providing a longer path for the heat flow and an increased heat exchange area so that the heat transfer from the inner shell to the outer shell is substantially reduced.
From the contact area of the support structure with the inner shell in the overlap area of the inner shell with the exhaust pipe, the inner shell is not in contact with the outer shell and terminates also in spaced relationship, preferably with a small axial distance from the downstream radially inner end of the outer shell. The end of the outer shell is preferably rounded, that is it is rolled in, or inwardly flanged. If, in this case, the inner shell is adapted in diameter to the open cross-section at the end of the outer shell and the inner shell forms practically an extension of the inner end of the exhaust pipe, that is, the inner shell is somewhat smaller in diameter than the inner end of the inwardly extending end flange of the outer shell in which the inner shell is disposed, an additional suction effect is provided. As a result, the air flowing through the air gap forms a protective envelop between the exhaust gas and the outlet area, whereby heating of the outer shell is reduced and contamination caused by the exhaust gas and also temperature-caused discoloration of the end of the outer sleeve are avoided. The arrangement according to the invention can be realized also with a relatively short inner shell. Extensions of the outer shell beyond the inner shell can be formed in a funnel-like fashion for collecting the airflow directed to the air gap.
It is further possible by an adequate designing of the support structure between the inner and the outer shells, for example by providing a support flange, which is oversized with respect to the outer shell, to mount the inner shell in the outer shell by axially pressing the flange into the outer shell with a press fit. A welding connection in the visible area of the exhaust pipe or the tail pipe can be avoided in this way.
Additional connections such as welding may then only be provided for safety reasons. Such a safety connection may be provided by a spot weld of one or several straps of the support structure.
Additional details and features of the invention will be described below on the basis of the accompanying drawings.